The 3rd Generation Partnership Project (3GPP) is a collaboration between groups of telecommunications associations, aiming to make a globally applicable third-generation (3G) mobile phone system specification. The 3GPP standardization encompasses Radio, Core Network and Service architecture. Examples of 3PP systems are Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), and Long Term Evolution (LTE).
The 3GPP system is currently being evolved into a system that is composed of heterogeneous access systems with the aim to provide various services at a higher-data-rate and lower-latency. Efficient resource usage, charging and international roaming, inherent features within the 3GPP system, are among the key capabilities to be retained in the evolved system. A policy control and charging (PCC) architecture allows operators and service providers to perform service based Quality of Service (QoS) policy and flow based charging control in communication networks. The level of service is related to the charged rates.
In telecommunication systems or networks signals are sent between network nodes. Bearers are used for transmitting the signals between the network nodes.
A Policy Control and Charging Rules Function (PCRF) is a functional element in communication networks that encompasses policy control decision and flow based charging control functionalities. The PCRF provides network control regarding the service data flow detection, gating, QoS and flow based charging, except credit management, towards a Policy and Charging Enforcement Function (PCEF). The PCRF receives session and media related information from an Application Function (AF) and informs the AF of traffic plane events.
The PCRF shall provision PCC Rules to the PCEF via a Gx reference point. A PCC Rule is a set of information enabling the detection of a service data flow and providing parameters for policy control and/or charging control
The PCRF shall inform the PCEF through the use of PCC rules on the treatment of each service data flow that is under PCC control, in accordance with the PCRF policy decision(s). The PCC rule comprises information for detecting the service data flow, for instance charging information and QoS information, e.g. Maximum Bit Rate (MBR), Guaranteed Bit Rate (GBR), QoS Class Identifier (QCI), Allocation, Retention and Priority (ARP).
Maximum Bitrate (MBR) defines the maximum number of bits delivered within a period of time, divided by the duration of the period. The Maximum bitrate is the upper limit a user or application may accept or provide. All bearer service attributes may be fulfilled for traffic up to the Maximum bitrate depending on the network conditions.
Guaranteed Bitrate (GBR) defines the guaranteed number of bits delivered within a period of time, provided that there is data to deliver, divided by the duration of the period. Bearer service attributes, e.g. delay and reliability attributes, are guaranteed for traffic up to the Guaranteed bitrate. For the traffic exceeding the Guaranteed bitrate the bearer service attributes are not guaranteed.
The AF is an element offering applications in which service is delivered in a different layer, i.e. transport layer, from the one the service has been requested, i.e. signaling layer, the control of Internet Protocol (IP) bearer resources according to what has been negotiated. One example of an AF is a Proxy Call Session Control Function (P-CSCF) of an IP Multimedia Core Network (IM CN) subsystem. The AF shall communicate with the PCRF to transfer dynamic session information, i.e. description of media to be delivered in the transport layer. This communication is performed using an Rx interface.
The PCEF encompasses service data flow detection, based on filters definitions included in the PCC rules, as well as online and offline charging interactions (not described here) and policy enforcement. Since the PCEF is the entity handling the bearers, it is where the QoS is being enforced for the bearer according to the QoS information coming from the PCRF. This functional entity is located at a Gateway, e.g. Gateway General packet radio service Support Node (GGSN) in the General Packet Radio Service (GPRS) case, Packet Data Network Gateway (PDN GW) in an LTE case, and Packet Data Gateway (PDG) in the Wireless Local Area Network (WLAN) case. For the cases where there is Proxy Mobile IP (PMIP) instead of GPRS Tunneling Protocol (GTP) protocol used between a Bearer Binding and Event Reporting Function (BBERF) and the PCEF, the bearer control is done in the BBERF instead, in this case QoS rules are provided to the BBERF.
Bearer binding as used in 3GPP is an association of the PCC rule and when applicable its QoS rule, if applicable, to an Internet Protocol Connectivity Access Network (IP-CAN) bearer within an IP-CAN session. For 3GPP Evolved Packet System (EPS) accesses, this function is located either at the BBERF or at the PCEF, depending on the deployment architecture. An IP-CAN bearer is an IP transmission path of defined capacity, delay and bit error rate, etc. An IP-CAN session is an association between a user equipment (UE) and an IP network. The association is identified by a UE IP address together with a user equipment identity information, if available. An IP-CAN session incorporates one or more IP-CAN bearers. An IP-CAN session exists as long as the UE IP address is established and announced to the IP network.
The BBERF/PCEF evaluate whether it is possible to use one of the existing IP-CAN bearers or not and whether to initiate IP-CAN bearer modification if applicable. If none of the existing bearers are possible to use, the BBERF/PCEF initiate the establishment of a suitable IP-CAN bearer. The binding is created between service data flow(s) and the IP-CAN bearer which have the same QCI and ARP, i.e. which have the same QoS categorization and priority demands.
In principle, it is assumed that all PCC rules with the same QCI and ARP are bound to the same bearer. From the standards point of view, it is not precluded to establish more than one bearer with the same QCI and ARP, although it is not specified under which conditions this could occur.
The MBR of the bearer is calculated as the sum of all the MBRs of the PCC rules bound to that bearer. In the same way, the GBR of the bearer is calculated as the sum of all the GBRs of the PCC rules bound to that bearer.
Until Release 10 of 3GPP standardization for communication networks, 3GPP networks with EUTRAN access have the limitation that the MBR of a particular bearer shall be set equal to the GBR. This limitation has been removed from Release 10 onwards. In principle, having MBR=GBR ensures that there will be no bandwidth limitation in the support of the services bound to a specific bearer. Since the network has reserved resources considering that the maximum bit rate that may be required for all services, the user experience will not be downgraded.
The decision to set MBR and GBR for the PCC rule is located in the PCRF and is based on operator policies, AF identifier and Session Description Protocol (SDP) negotiated information, for IP Multimedia Subsystem (IMS) services. SDP is a format for describing multimedia communication sessions, such as streaming media initialization parameters or multicast session setup. IMS is an architectural framework for delivering IP multimedia services.
Explicit Congestion Notification (ECN) is an extension to LP and Transmission Control Protocol (TCP) protocols that allow end-to-end notification of network congestion without dropping packets. In order to support this feature, it is needed that both end points support it.
Traditionally, TCP/IP networks signal congestion by dropping packets. When ECN is successfully negotiated, an ECN-aware node may set a mark in the IP header instead of dropping a packet in order to signal impending congestion.
At the receiving end point this congestion indication is handled by an upper layer protocol and needs to be echoed back to the transmitting node in order to reduce its required transmission rate, for instance by changing media encoding.
3GPP has agreed on a mechanism to notify the data source that there is a risk for congestion in the network using Explicit Congestion Notification.
Using ECN, when Real-Time Transport Control Protocol (RTCP) is used, comprises feedback of ECN congestion experienced markings to the sender using RTCP, verification of ECN functionality end-to-end and how to initiate ECN usage. The initiation process impacts the signaling mechanism using SDP. Having ECN as part of IMS solutions, allows minimizing the impact of congestion on real-time multimedia traffic.
When ECN is supported, the Evolved Universal Terrestrial Radio Access Network (EUTRAN) should attempt not to drop any packets on a bearer during a defined grace period. It is considered that in this case the MBR of the bearer may be greater than the GBR. Since the PCRF is the node that sets both the MBR and GBR, it is assumed that it has the knowledge about the network support of this feature. The PCRF sets MBR>GBR as appropriate for flows as defined for IMS.
The handling of a rule with MBR>GBR is up to operator policy, e.g. an independent IP-CAN bearer may be maintained for that SDF to prevent unfairness between competing SDFs.
As described in previous clauses, the PCRF may decide to set the MBR greater than the GBR for the applicable service data flows in certain cases, comprising when ECN is supported.
It is also indicated that it may be assumed that the normal case will be that only one bearer is established for all service data flows with the same QCI and ARP. In principle there is no need to establish a second bearer when the MBR=GBR for all flows bound to that bearer, since the resources, i.e. GBR, are reserved considering the maximum bandwidth allowed for each flow, i.e. MBR. Each flow is also policed for its MBR which ensures that the flow does not interfere with the GBR granted for other flows. It should be noted that ECN is signaled to all sources, i.e. all service flows on the same bearer may be affected.
When ECN is supported and flows where MBR>GBR are provided, if only one bearer with the same QCI/ARP is established, the codec rate, encoding, of certain services will be reduced when a congestion situation is detected. The user perception will then be affected for those services that are marked with the ECN congestion indicator.
If the PCRF establish the MBR>GBR based on other operator policies, without the ECN support, the packets for certain services may be dropped, and then the user perception will be worse.
This congestion situation could be solved by always establishing a separate bearer whenever a service requires MBR>GBR. However, there is a limitation to the number of bearers in the UE access to the public land mobile network (PLMN). Mandating a separate bearer for each flow with MBR>GBR increases the signaling and may cause the number of possible bearers being exhausted.
In 3GPP and in bilateral discussions it is declared that the legacy principle with one bearer per QCI/ARP combination suffice when either (a) all the PCC rules have MBR equal to the GBR, or (b) there is full support for ECN for all the PCC rules where MBR is greater than GBR.
The 3GPP standard however does not prevent, and should not prevent, that a PCC rule, based on operator policy, gets an MBR greater than GBR without the support for ECN being present. For such use, there is a risk for unfairness between flows when allocated to the same bearer. In order to avoid possible unfairness, each such flow could get a separate bearer. For the case of full support for ECN, it would however generate unnecessarily many bearers.